Continuing with our new #AMbasics series, which started with polymer material properties, this post centers around additive manufacturing (AM) metal material properties in general and they are illustrated through metal powder AM. To this end, I interviewed Sascha Rudolph, Commercial Director Metal Materials at EOS. Sascha heads the business aspects of this relatively new EOS business unit. In general, when it comes to metal material properties, AM is very different than other manufacturing methods or as Sascha puts it: “We aren’t casting, we’re micro-welding. In AM we aren’t melting at once and solidifying at once. Instead, in AM, the powder is melted in a small area, the laser moves and the next small area is melted as the previous one starts cooling. This is repeated millions of times.” This means that the process affects the results and therefore properties vary. “It’s really important to differentiate between powder properties and part properties” Sascha says. While the powder metal material properties are uniform (and tested by the supplier) they are not the same as the part properties – these depend on the AM process used (which includes the specific model of printer, and the settings the printer uses) as well as the heat treatment applied when printing is done. “If people are used to using a 6061 casting alloy that gives them certain properties, they can expect an additive alloy to give them 6061 properties but they cannot expect it to be the same alloy (raw powder) as they used for casting” explains Sascha. Different machines, different settings, different heat treatments will all result in different metal material properties in the final part. This is why EOS gives datasheets for the trio of powder, machine model (+settings), and heat treatment. The datasheets also show how homogeneous the crystal structures are – homogeneous structures mean that if you lock in this trio (LEO Lane can help with that) you will get repeatable metal material properties. Below, we will go over some of the basic terms (some will be repeated from the polymer post in this series, and I will point out the differences, if any) followed by some basic (and slightly less basic) metal material properties. Sascha illustrated the properties we discussed through the data sheet for a Nickel Alloy (the illustrations below are from EOS photo and this datasheet).
Basic Terms Used
Stress – internal force particles exert on each other (e.g., when a sample is bent)
Strain – the deformation of a sample (how much it is displaced from its original shape)
Pascal – The pascal (Pa) is a unit of pressure used to quantify internal pressure, stress, Young’s modulus, and ultimate tensile strength. A standard atmosphere is 101,325 Pa. MPa denotes mega pascal (1,000,000 pascals).
HRC – Hardness Rockwell C scale (there is also a Hardness Rockwell B scale but it is less used). The numbers are relative to this scale in HRC units.
Basic Mechanical AM Metal Material Properties (and Their Tests)
How hard it is to indent a sample. The test is using a diamond tip seeing how much force is needed to puncture the sample. It is measured in HRC. If you increase mechanical hardness, the material becomes more brittle.
Strength (Ultimate Tensile Strength)
Strength is the force you need to pull a material in opposite directions in order to break it. Tensile strength is measured in pascals (Pa).
Yield Strength is the stress level at which a predetermined amount of permanent deformation occurs. Yield strength is measured in MPa.
Elongation is a measure of how elastic the part is. How much it elongated under a predetermined load. There is also an Elongation to Break test which checks how much a sample stretches before it breaks (measured in %: the ratio of elongated sample just before breaking to its original length).
The density of the material compared with water density (water density = 1). Another measure of density is based on porosity: the item is weighed in air and in water or another liquid (by suspending it from a scale in both mediums) and the density is calculated from those numbers.
More AM Metal Material Properties
Fatigue is how quickly the part wears down with repeated use. The measure is based on repeated tests at a constant strain to see how many cycles it takes to break the part. It is performed at a specific temperature (the test is repeated in different temperatures). It is measured in number of cycles until breakage. This is a very expensive test because keeping the temperature high requires energy and because of the many cycles it is time and energy consuming.
Creep is the deformation over time when a constant load is applied at a constant temperature. This can indicate how robust to deformation a part will be in its intended temperature range and surrounding forces. Creep is measured in LMP (Larson Miller Parameter) which takes into account both temperature and the time to rupture for a given load. Above you can see the creep of EOS’ Nickel Alloy on the M404 machine at 40μm (line) compared to the creep of casted nickel alloy (the light blue area).
As usual, there is room above to add more properties and tests as needed – let me know if I missed something and stay tuned for more #AMbasics posts by following us on LinkedIn or subscribing to our newsletter for weekly updates and to make sure you don’t miss a post. Top photo: looking for attribution (please advise if it is you).